The use of gas turbine engines in automotive and other ground transportation applications necessitates control devices and designs differing from those used with gas turbine aircraft engines, since a gas turbine engine used in ground transportation vehicles must operate under a unique set of conditions and with a type of response constituting an analog of the traditional piston engine-powered vehicle performance. In particular, one of the control functions important to the operation of a ground transportation gas turbine engine is the positioning of the power turbine nozzles, which are adjusted to vary the speed of the turbine output.
The input electrical signal is produced by a preprogrammed control computer operating in response to various control and condition parameters such as accelerator pedal position, ambient temperature, ambient pressure, gas generator speed, gas generator turbine temperature, regenerator "hot side temperature", and transmission output shaft velocity. A servoactuator device is utilized to produce an output motion in proportion to the input electrical control signal.
The output motion of the servoactuator device acts through a suitable linkage mechanism to drive a ring gear, which in turn rotates the power turbine nozzles through the desired angular travel. Particular angular settings of the nozzles are specified during acceleration, deceleration, start-up, and steady-state operation of the gas turbine engine. The nozzles may be positioned in a braking mode by the servoactuator device so that some degree of braking of the vehicle is attained from the gas turbine engine.
Computer control systems to operate the system in accordance with the signals provided by the various sensors are known in the art, and do not comprise any portion of the present invention. The main difficulty in developing a satisfactory control system for use with ground transportation gas turbine engines has been in designing a suitable servoactuator device to control the nozzle position. The most practical servoactuator device to date is described in U.S. Pat. No. 4,044,652, assigned to the assignee of the present invention, and that patent is hereby incorporated herein by reference. The device disclosed therein is a hydraulic motor having an output shaft for coupling to the ring gear to position the turbine nozzle. Movement of the hydraulic motor is controlled by a hydraulic servo valve actuated by a proportional solenoid. Since motion of the hydraulic motor piston causes a corresponding translation of the solenoid and valve, mechanical feedback obtained therein linearizes the response function to eliminate the need for closed loop operation of the system in which the servoactuator is used.
Although the servoactuator device disclosed in U.S. Pat. No. 4,044,652 provides some significant advantages over other servoactuator devices known, it has, like these other devices, a number of significant problems. One of these problems stems from the desirability of obtaining a deceleration mode in which the servoactuator causes the nozzles to be reversed to cause the turbine engine to produce a braking effect upon the vehicle. To attain a braking mode in devices such as that disclosed in the above-referenced U.S. patent, it is necessary to operate the servoactuator so that it will be positioned in the reverse mode when the input electrical control signal provided to the servoactuator is at zero. As the input electrical control signal increases, the servoactuator will operate to move the nozzles to the zero degree angular position, and then to a positive angular position.
The result of this type of operation is that in order to position the nozzles in the zero degree angular position, the electrical control signal must be at a significant portion of its maximum value, such as one-half of maximum value. During operation of the turbine in the forward mode, in order to reverse the nozzles and position them in the braking mode, that is, in a negative angular position, the electrical control signal must be changed from a value greater than half of the maximum control signal to a level very near to a zero signal. It would be more advantageous to have a servoactuator which was at the zero degree angular position when the electrical control signal was also at a zero level, the angular position increasing in a positive manner as the electrical control signal was increased.
In addition, instead of operating the braking mode in the manner just described, it would be desirable to provide the servoactuator device with a toggling action, so that when the electrical control signal reaches a certain predetermined positive value, the servoactuator will immediately position the nozzles in the negative angular position, thus causing dynamic braking of the vehicle. In order to attain such operation from the device described in the above-referenced U.S. patent, a dump valve would have to be installed in the system; such a valve may give some degree of toggling effect, but at the price of an additional component which increases the cost, weight, and space requirements of the control system. Therefore, a servoactuator device which incorporates the toggling effect without the requirement of the additional components is highly desirable.
Servoactuator devices presently existing all share another major problem-adverse consequences in the operation of the servoactuator following either an electrical failure or a hydraulic failure in the control system. In the event of either type of failure, it is highly desirable that the servoactuator cause the nozzles to be positioned in the zero degree angular position to prevent the turbine engine from being damaged or placed in a runaway mode.
In the event of an electrical failure, the system described above will cause the nozzles to be placed in the full negative angular position to cause dynamic braking of the vehicle. Depending on the condition the turbine engine was operating in when the electrical failure occurred, such a failure could have catastrophic results, possibly resulting in the destruction of the turbine engine. Even should the result be less severe, there would be a tendency for the nozzles to direct the gas in a way tending to impart a reverse velocity to the power turbine, a highly undesirable effect.
The effects of the hydraulic failure are equally dissettling, since with a loss of hydraulic pressure the servoactuator will tend to stay in the position it was in when the failure occurred. Therefore, if the turbine engine was being operated in an acceleration mode at the time of hydraulic failure, the nozzles will remain in the acceleration position, possibly causing a runaway condition to occur in the turbine engine. Therefore, it can be seen that it is desirable in the event of either an electrical failure or a hydraulic failure, for the servoactuator to operate in a failure mode in which the nozzles are positioned in the zero degree angular setting.